2 We are very much indebted to Milton Rosenbaum for building the apparatus and constructing the stimulus materials.

3 studies dealt with the attainment of disjunctive concepts. The positive instances of disjunctive concepts contain either Values A or B, while conjunctive concepts are those whose positive instances contain both A and B. The former are typically more difficult to attain. The 1st experiment showed that training on disjunctive concept problems increases the proportion of Ss who offer a disjunctive concept when a conjunctive concept is also available. The 2nd experiment compared inclusive (A or B or both) and exclusive (A or B but not both) disjunctive problems. The latter were found to be more difficult in terms of the time to solution.Several recent studies have investigated the influence of the "logical structure" of the concept on the process of concept formation. Concept formation studies have typically used conjunctive concepts, in which all positive instances contain a common combination of elements (e.g., red circles); and relational concepts, 1 This research is based on a dissertation submitted to Yale University in partial fulfillment of the requirements for the PhD degree. The author was supported by a United States Public Health Service Predoctoral Research Fellowship.The author wishes to express a special debt to the late Carl I. Hovland, who served as the original chairman of his advisory committee. His suggestions and critical comments contributed in large part to the general form of the study. The author is also indebted to the members of his committee, Donald W. Taylor (who served as chairman during the final stages of the work), Sidney J. Blatt, and William Kessen, for their generous help and guidance.Support for the preparation of this report for publication was provided by a grant from the Ford Foundation to Donald W. Taylor for his continuing program of research on thinking.The author wishes to thank Paul A. Lane, Richard Ehmer, and Marjorie Ehmer of the University of Bridgeport Psychology Department for their help in recruiting 5s and in providing space for the experimental sessions.2 Now at the University of Washington.

The length of the intertrial interval (ITI) was found to be positively related to the efficiency of concept learning in a blank-trials task. This effect was mediated primarily by the effect of ITI length on immediate information processing (local consistency). There was no discernable effect on memory for prior information or on the tendency to respond according to an identifiable hypothesis. ITI length had little if any effect on win-stay, lose-shift performance.

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